Buy a black light. Black lights can be obtained online and come in a variety of sizes, from flashlights and bulbs to hand held UV wands, such as those seen on crime scene TV shows. With the hand-held wands, children taking part in the experiment can check out the resulting glow more closely than with a bulb.
Buy a glow-in-the-dark lotion especially designed for experiments. You can find these lotions online. Use petroleum jelly as an alternative.
Pick a volunteer from the group. Cover his hands thoroughly in the glow lotion and tell the children the lotion represents germs such as a cold virus. Turn off the lights and close the blinds. Turn on the UV light. Show the other children how the lotion on the hands of the volunteer glows under the ultraviolet light.
Tell the volunteer to shake hands with another member of the group. Then have that child shake hands with another group member, and repeat until all group members have participated. Turn off the lights again and turn on the UV light. Begin with the glowing hands of the first child and follow down the chain of hand-shakers to show how the "germs" can be transferred from person to person.
Allow all of the children to cover their hands in the lotion. Show them the glow of their unwashed hands. Then tell them to wash their hands and show them the result of the hand-washing. Point out the parts of their hands that they didn't clean sufficiently to remove the glow. This is a handy way of demonstrating good hygiene practices.
Allow the children to cover their hands in lotion again. Go about the daily business of the class, and at the end of the day, bring out the UV light again, or use a UV bulb this time to illuminate the classroom. During the day, the kids should have been touching faces, chairs, windows and books, and the resultant glow will be both entertaining and educative, both for you and for the children.